A method of producing SOI wafers by separating ions-implanted wafers after bonding, i.e., the ion implantation separation method (an art also referred to as the Smart Cut method (registered trademark)), has attracted attention as a method for producing SOI wafers.
In this method, for example, an oxide film is formed on at least one of two silicon wafers, and gas ions, such as hydrogen ions or rare gas ions, are implanted from a front surface of one of the silicon wafers (a bond wafer) to form a layer of the implanted ions (also referred to as a micro bubble layer) in the interior of the wafer.
The oxide film is then interposed between the surface from which the ions have been implanted and the other silicon wafer (a base wafer) so that these wafers are bonded. These wafers after bonding are then subjected to a heat treatment in which the wafers are left in a heat treatment furnace at a given temperature for a prescribed time after the temperature was increased at a rate of about 10° C. per minute, so that the bond wafer is cleaved along the layer of the implanted ions, whereby a bonded wafer is formed. A heat treatment (a bonding heat treatment) is then performed on this bonded wafer at a high temperature to strengthen a bond between the base wafer and an SOI layer separated from the bond wafer. In this way, an SOI wafer is obtained (See Patent Document 1, for example).
In production of a bonded wafer by the ion implantation separation method, the bond wafer used to form a thin film usually has the same plane orientation as the base wafer used as a support substrate. Wafers having a cutout portion such as a notch or orientation flat that shows the plane orientation at the same position are used to form the SOI wafer by bonding through the oxide film or a directly bonded wafer by bonding without an oxide film.
The bond wafer and the base wafer are usually bonded at room temperature such that the cutout portion of these wafers is aligned, and the resultant wafers are then subjected to a separation heat treatment to separate the bond wafer along the layer of the implanted ions, whereby the bonded wafer is produced.